<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script language="JavaScript">
      function completeAndReturnName() {
          var fname=document.getElementById('fname').value;
          var lname=document.getElementById('lname').value;
          var  full=fname+' '+lname;
          document.getElementById('fullname').value=full;
          document.getElementById('submit-btn').style.display='block';
          return full;

      }

  </script>
</head>
<body>
  hellp  pyqt

  <form>
      <label for="fname">First name:</label>
      <input type="text" name="fname" id="fname"/>
      <br>
      <label for="lname">Last name:</label>
      <input type="text" name="lname" id="lname"/>
      <br>
      <label for="fullname">Full name:</label>
      <input disabled type="text" name="fullname" id="fullname"/>
      <br>
      <input style="display: none" type="submit"  id="submit-btn"/>


  </form>
</body>
</html>